Re: [Qemu-devel] reduce write bandwidth of qcow2 driver while allocating new cluster

[Eric Blake]

[adding qemu-block]

On 08/28/2017 12:56 AM, Liu Qing wrote:
> Dear list,
>     Recently I used fio to test qcow2 driver in the guest os, and found out
> that when a new cluster is allocated the 4K IO will occupy 64K(default cluster
> size) bandwith.
>     From the code qcow2 driver will fill the unused part of new allocated
> cluster with 0 in perform_cow. These 0s are set in qcow2_co_readv when the 
> read
> destination is not allocated and it has no backing file. Could I forbidden any
> further write in copy_sectors if the copy source is not allocated and it has
> no backing file? So only the requested data is written to the cluster. 
> Function
> copy_sectors is only used by perform_cow in the master branch.

There have already been discussions on optimizing COW writes in a manner
similar to what you are describing; for example,

https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg00109.html

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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